Engineering and Construction

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During construction, excavations must be kept free of standing water.
Such water may come from precipitation or it may come from ground-water seepage originating from any of a number of sources, such as surface water percolating through the soil, underground streams, perched water moving over impervious soil strata, or adjacent permanently sat-urated soil areas where the excavationextends below the water table.

Some shallow excavations in relatively dry soil conditions may remain free of standing water without any intervention. But most excavations require some form of dewatering, or extraction of water from the excavation or surrounding soil. The most common method of dewatering is to remove water by pumping as it accumulates in pits, called  sumps, created at low points in the excavation. Where the volume of ground-water flowing into the excavation is great, or with certain types of soils, particularly sands and silt, that may be softened by constant seepage, it may be necessary to keep ground-water from entering the excavation at all. This can be done either by pumping water from the surround-ing soil to depress the water table below the level of the bottom of the excavation or by erecting a watertight barrier, such as a slurry wall, around the excavation (Figure 2.26).

Figure 2.26 Two methods of keeping an excavation dry, viewed in cross section. The water sucked from well points depresses the water table in the immediate vicinity to a level below the bottom of the excavation. Watertight barrier walls work only if their bottom edges are inserted into an impermeable stratum that prevents water from working its way under the walls.

 Well points are commonly used to depress the water table. These are vertical sections of pipe with screened openings at the bottom that keep out soil particles while al-lowing water to enter. Closely spaced well points are driven into the soil around the entire perimeter of the excavation. These are connected to horizontal header pipes leading to pumps that continually draw water from the system and discharge it away from the building site. Once pumping has drawn down the water table in the area of the excavation, work can continue “in the dry” (Figures 2.27 and 2.28). For excavations deeper than the 20 feet (6 m) or
so that cannot be drained by a suction pump stationed at ground level, two rings of well points may be required, the inner ring being driven to a deeper level than the outer ring, or a single ring of deep wells with submersible force pumps may have to be installed.

Figure 2.27 Well point dewatering. Closely spaced vertical well points connect to the larger-diameter header pipe.

Figure 2.28 An excavation is kept dry despite the close proximity of a large body of water. Two dewatering pumps are visible in the foreground.  A pair of header pipes and numerous well points can be seen surrounding the excavation.

In some cases, well points may not be practical: they may have insufficient capacity to ensure that an ex-
cavation remains dry; restrictions on the disposal of groundwater may limit their use; reliability due to power outages may be a concern; or lowering of the water table may have serious adverse effects on neighboring buildings by causing consolidation and settling of soil under their foundations or by exposing untreated wood foundation piles, previously protected by total immersion in water, to decay. In these cases, a watertight barrier wall may be used as an alternative (Figure 2.26). Slurry walls and soil mixed walls (pages 40–45) can make excellent watertight barriers. Sheet piling can also work, but it tends to leak at the joints.  Soil freezing is also possible. In this method, an array of vertical pipes similar to well points is used to continuously circulate coolant at temperatures low enough to freeze the soil around an excavation area, resulting in a temporary but reliable barrier to groundwater. Watertight barriers must resist the hy-drostatic pressure of the surrounding water, which increases with depth, so for deeper excavations, a system of bracing or tiebacks is required. A watertight barrier also works only if it reaches into a stratum of impermeable soil such as clay.
Otherwise, water can flow beneath the barrier and rise up into the excavation.

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If the site is sufficiently larger than the area to be covered by the building, the edges of the excavation can be sloped back or benched at an angle such that the soil will not slide back into the hole. This angle, called the angle of repose, can be steep for cohesive soils such as the stiffer clays, but it must be shallow for frictional soils such as sand and gravel. On constricted sites, the soil surrounding an excavation must be held back by some kind of  slope support or  excavation support capable of resisting the pressures of earth and groundwater (Figure 2.10). Such construction can take many forms, depending on the qualities of the soil, depth of excavation, equipment and preferences of the contractor, proximity of sur-rounding buildings, and level of the water table.

Figure 2.10 On a spacious site, an excavation can be benched. When excavating close to  property lines or nearby buildings, some form of slope support, such as sheeting,  is used to retain the soil around the  excavation.

Shoring
The most common types of slope sup-port, or  shoring, are soldier beams and lagging, and sheet piling. With soldier beams and lagging, steel columns called  H-piles or  soldier beams are driven vertically into the earth at  small intervals around an excavation site before digging begins. As earth is  removed, the  lagging, usually consisting of heavy wood planks, is placed against the flanges of the columns to retain the soil outside the excavation (Figures 2.11 and 2.12).  Sheet piling or  sheeting consists of vertical planks of wood, steel, or precast concrete that are placed tightly against one another and driven into the earth to form a solid wall before excavation begins (Figures 2.13 and 2.14). Most often, shoring is temporary, and it is removed as soil is replaced in the ex-cavation. However, it may also be left in place to become a permanent part of the building’s substructure. This may be necessary, for example, where shoring is located extremely close to a property line and there is no practical way to remove it after completion of construction without disturbing adjacent property or structures.

Figure 2.11 Soldier beams and lagging, seen in horizontal section.

Figure 2.12 Soldier beams and lagging. Lagging planks are added at the bottom as excavation proceeds. The drill rig is boring a hole for a tieback to brace a soldier beam.

Figure 2.13 Horizontal sections through three types of sheet piling. The shading represents the retained earth.

Figure 2.14 Drilling tieback holes for a wall of steel sheet piling. Notice the completed tieback connections to the horizontal waler in the foreground. The hole in the top of each piece of sheet piling allows it to be lifted by a crane.

Slope support may also take the form of  pneumatically applied concrete, also called  shotcrete, in which excavation proceeds first and then the sloped sides are reinforced with a relatively stiff concrete mixture sprayed directly from a hose onto the soil.


This method works well where the soil is sufficiently cohesive to  hold a steep slope at least temporarily.

The hardened concrete reinforces the slope and protects against soil erosion. (Figure 2.15).

Figure 2.15 Where slope support turns the corner in this excavation and the soil can be sloped at a lesser angle, less expensive shotcrete takes the place of soldier beams and lagging.

Slurry Walls
A slurry wall is a more complicated and expensive form of excavation support that is usually economical only if it becomes part of the permanent foundation of the building. The first steps in creating a slurry wall are to lay out the wall’s location on the surface of the ground with surveying instruments and to define the location and thickness of the wall with shallow poured concrete guide walls (Figures 2.16 and 2.17). When the formwork has been removed from the guide walls, a special narrow  clamshell bucket, mounted on a crane, is used to excavate the soil from between the guide walls. As the narrow trench deepens, the tendency of its earth walls to collapse is counteracted by filling the trench with a viscous mixture of water and bentonite clay, called a slurry, which exerts  pressure  against the earth walls, holding them in place. The clamshell bucket is lowered and raised through the slurry to continue excavating the soil from  the bottom of the trench until the desired depth has been reached, often a number of stories below the ground.


Slurry is added as required to keep the trench full at all times.

Figure 2.16 Steps in constructing a slurry wall.  (a) The concrete guide walls have been installed, and the clamshell bucket  begins excavating the trench through a bentonite clay slurry. (b) The trench is  dug to the desired depth, with the slurry serving to prevent collapse of the walls  of the trench. (c) A welded cage of steel reinforcing bars is lowered into the  slurry. (d) The trench is concreted from the bottom up with the aid of a tremie.  The displaced slurry is pumped from the trench, filtered, and stored for reuse.  (e) The reinforced concrete wall is tied back as excavation progresses.

Figure 2.17 Constructing a slurry wall. (a) The guide walls are formed and poured in a shallow trench. (b) The narrow clamshell bucket  discharges a load of soil into a waiting dump truck. Most of the trench is covered with wood pallets for safety.

Meanwhile, workers have welded together cages of steel bars designed to reinforce the concrete wall that will replace the slurry in the trench. Steel tubes whose diameter corresponds to the width of the trench are driven vertically into the trench at predetermined intervals to divide it into sections of a size that can be reinforced and concreted conveniently. The concreting of each section begins with the lowering of a cage of reinforcing bars into the slurry. Then concrete is poured into the trench from the bottom up, using a funnel-and-tube arrangement called a  tremie. As the concrete rises in the trench, it displaces the slurry, which is pumped out into holding tanks, where it is stored for reuse. After the concrete reaches the top of the trench and has hardened sufficiently, the vertical pipes on either side of the recently poured section are withdrawn from the trench, and the adjoining sections are poured. This process is repeated for each section of the wall. When the concrete in all the trenches has cured to its intended strength, earth removal begins inside the wall, which serves as sheeting for the excavation.


In addition to the sitecast concrete slurry wall described in the preceding paragraphs, precast concrete slurry walls are built. The wall is pre-stressed and is produced in sections in a precasting plant (see Chapter 15), then trucked to the construction site. The slurry for precast walls is a mixture of water, bentonite clay, and portland cement. Before a section is lowered by a crane into the slurry, its face is coated with a compound that prevents the clay–cement slurry from adhering to it (Figure 2.18). The sections are installed side by side in the trench, joined by tongue-and-groove edges or synthetic rubber gaskets. After the portland cement has caused the slurry to harden to a soillike con-sistency, excavation can begin, with the hardened slurry on the inside face of the wall dropping away from the coated surface as soil is removed.

Figure 2.18 Workers apply a nonstick coating to a section of precast concrete slurry wall as it is lowered into the trench.

Figure 2.19 Soil mixing.

The primary advantages of a precast slurry wall over a sitecast one are better surface quality, more accurate wall alignment, a thinner wall (due to the structural efficiency of prestressing), and improved watertightness of the wall because of the continuous layer of solidified clay outside.

Soil Mixing
Soil mixing is a technique of adding a modifying substance to soil and blending it in place by means of augers or paddles rotating on the end of a vertical shaft (Figure 2.19). This technique has several applications, one of which is to remediate soil contaminated with a chemical or biological substance by blending it with a chemical that renders it harmless. Another is to mix portland cement and water with a soil to create a cylinder of low-strength concrete in the ground. A linear series of these cylinders can serve as a cutoff wall against water penetration or as excavation support (Figure 2.20). Soil mixing can also serve to stabilize and strengthen areas of weak soil.

Figure 2.20 Soil-mixed excavation support.  (a) Excavation proceeds after completion of soil mixing. Bracing  consists of soldier piles, walers, and tiebacks. The soldier piles are inserted  during soil mixing, before the soil/ cement mixture hardens. The walers and  tiebacks are installed later, as excavation progresses. (b) Fully excavated soil- mixed sheeting. This excavation support system must be strong enough to resist  the soil pressures caused by the adjacent  buildings.

Bracing
All forms of slope support and excavation support must be braced against soil and water pressures as the excavation deepens (Figure 2.21).

Figure 2.21 Three methods of bracing shoring drawn  in cross section. The connection between the waler and the brace, raker, or tieback  needs careful structural design. The broken line between rakers indicates  the mode of excavation: The center of the hole is excavated first with sloping  sides, as indicated by the broken line. The heel blocks and uppermost tier of rakers are installed. As the sloping sides are excavated deeper, more tiers  of rakers are installed. Notice how the tiebacks leave the excavation totally free  of obstructions.

Crosslot bracing utilizes temporary steel wide-fl ange columns that are driven into the earth by a piledriver at points where braces will cross. As the earth is excavated down around the sheeting and the columns, tiers of horizontal bracing struts, usually of steel, are added to support walers,  which are beams that span across
the face of the sheeting. Where the  excavation is too wide for crosslot bracing, sloping  rakers are used in-
stead, bearing against  heel blocks or other temporary footings.

Both rakers and crosslot bracing,  especially the latter, are a hindrance  to the excavation process. A clam- shell bucket on a crane must be used  to remove the earth between the  braces, which is much less effificient and more costly than removing soil with a shovel dozer or backhoe in an  open excavation.

Where subsoil conditions permit, tiebacks can be used instead of braces  to support the sheeting while main- taining an open excavation. At each  level of walers, holes are drilled at in-tervals through the sheeting and the surrounding soil into rock or a stra-tum of stable soil. Steel cables or ten-dons are then inserted into the holes, grouted to anchor them to the rock or soil, and stretched tight with hy-draulic jacks (posttensioned) before they are fastened to the walers (Figure 2.22 , Figure 2.24).

Figure 2.22 Three steps in the installation of a  tieback to a soil anchor. (a) A rotary drill bores a hole through the sheeting  and into stable soil or rock. A steel pipe casing keeps the hole from caving in  where it passes through noncohesive soils. (b) Steel prestressing tendons are  inserted into the hole and grouted under pressure to anchor them to the soil.  (c) After the grout has hardened, the tendons are tensioned with a hydraulic  jack and anchored to a waler.

Figure 2.24 Slurry wall and tieback construction used to support historic buildings around a deep excavation for a station of the Paris  Metro.

Excavations in fractured rock can  often avoid sheeting altogether, ei- ther by injecting grout into the joints of the rock to stabilize it or by drilling  into the rock and inserting  rock anchors that fasten the blocks together  (Figure 2.25).

In some cases, vertical walls of  particulate soils can be stabilized by  soil nailing. A soil nail is similar to a
rock anchor: It is a length of steel re- inforcing bar that is inserted into a  nearly horizontal hole drilled deep  into the soil. Grout is injected into the hole to bind the soil nail to the  surrounding soil. Large numbers of
closely spaced nails are used to knit  a large block of soil together so that  it behaves more like weak rock than  particulate soil.

Bracing and tiebacks in excavations are usually temporary. Their  function is taken over permanently  by the floor structure of the basement levels of the building, which is designed specifically to resist lateral  loads from the surrounding earth as well as ordinary floor loads.

Figure 2.25 Rock anchors are similar to tiebacks but are used to hold jointed rock formations in place around an excavation.

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At least some excavation is required for every building. Organic topsoil is subject to decomposition and to
shrinking and swelling with changes in moisture content. It is excellent for growing lawns and landscape plants but unsuitable for supporting buildings. Often, it is scraped away from the building area and stockpiled to one side for redistribution over the site after construction of the building is complete. After the topsoil has been removed, further digging is necessary to place the footings out of reach of water and wind erosion. In colder climates, foundations must be placed below the level to which the ground freezes in winter, the frost line, or they must be insulated in such a way that the soil beneath them cannot freeze.

Otherwise, a foundation can be lifted and damaged by soil that expands slightly as it freezes. Or, under certain soil and temperature conditions, upward migration of water vapor from the pores in the soil can result in the formation of  ice lenses, thick layers of frozen water crystals than can lift foundations by even larger amounts.

Excavation is required on many sites to place the footings at a depth where soil of the appropriate bearing capacity is available. Excavation is frequently undertaken so that one or more levels of basement space can be added to a building, whether for additional habitable rooms, for parking, or for mechanical equipment and storage. Where footings must be placed deep to get below the frost  line or reach competent soil, a basement is often bargain-rate space, adding little to the overall cost of the building.

In particulate soils, a variety of excavating machines can be used to loosen and lift the soil from the ground: bulldozers, shovel dozers, backhoes, bucket loaders, scrapers, trenching machines, and power shovels of every type. If the soil must be moved more than a short distance,
dump trucks come into use.

In rock, excavation is slower and many times more costly. Weak or highly fractured rock can sometimes be broken up with power shovels, tractor-mounted rippers, pneumatic hammers, or drop balls such as those used in building demolition. Blasting, in which explosives are placed  and detonated in lines of closely spaced holes drilled deep into the rock, is often necessary. In devel-oped areas where blasting is impractical, rock can be broken up with hydraulic splitters.

⇒ Excavation Support   If the site is sufficiently larger than the area to be covered by the building, the edges of the excavation can be sloped back or benched at an angle such that the soil will not slide back...

⇒ Dewatering   During construction, excavations must be kept free of standing water. Such water may come from precipitation or it may come from ground-water seepage originating from any of...

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a. Piping in sand.  The base of an excavation in sand is usually stable unless an unbalanced
hydrostatic head creates a "quick" condition.  Among the methods to eliminate instability are dewatering,
application of a surcharge load at the bottom of the excavation, and deeper penetration of the piling.

b. Heaving in clays.   The stability against heave of the bottom of an excavation in soft clay may be
evaluated from figure 14-12.  If the factor of safety is less than 1.5, the piling should be extended below the base of the excavation.  Heave may also occur because of unrelieved hydrostatic pressures in a permeable layer located below the clay.

c. Care of seepage.   Small amounts of seepage into the excavation can be controlled by pumping from sumps.  Such seepage can be expected if the excavation extends below the water table into permeable soils.  If the soils consist of fine sands and silts, the sumps should be routinely monitored for evidence of fines being washed from the soil by seepage.  If large quantities of fine-grained materials are found in the sumps, precautionary steps should be taken to make the lagging or sheeting watertight to avoid excessive settlements adjacent to the excavation.

Figure 14-12.  Stability of bottom of excavation in clay.

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a. Empirical design earth pressure diagrams developed from observations are shown in figure 14-11.

In soft to medium clays, a value of m = 1.0 should be applied if a stiff stratum is present at or near the base of the excavation.  If the soft material extends to a sufficient depth below the bottom of the excavation and significant plastic yielding occurs, a value of 0.4 should be used form.  The degree of plastic yielding beneath an excavation is governed by the stability number N expressed as


where  γ, H, and su, are defined in figure 14-11.  If N exceeds about 4, m < 1.0.

Figure 14-11.  Pressure distribution-complete excavation.

b. For stiff-fissured clays, diagram (c) of figure 14-11 applies for any value of N.  If soft clays, diagram (b) applies except when the computed maximum pressure falls below the value of the maximum pressure in diagram (c).  In these cases, generally for N < 5 or 6, diagram (c) is used as a lower limit.  There are no design rules for stiff intact clays and for soils characterized by both c and  φ such as sandy clays, clayey sands, or cohesive silts.

c. The upper tier of bracing should always be installed near the top of the cut, although computations may indicate that it could be installed at a greater depth.

Its location should not exceed 2su below the top of the wall.

d. Unbalanced water pressures should be added to the earth pressures where the water can move
freely through the soil during the life of the excavation.

Buoyant unit weight is used for the soil below water.

Where undrained behavior of a soil is considered to apply, the use of total unit weights in calculating earth
pressures automatically accounts for the loads produced by groundwater (fig 14-11).  Pressures due to the
surcharge load are computed as indicated in previous sections and added to the earth and water pressures.

e. Each strut is assumed to support an area extending halfway to the adjacent strut (fig 14-11).  The strut load is obtained by summing the pressure over the corresponding tributary area.  Temperature effects, such as temperature increase or freezing of the retained material,  may significantly increase strut loads.

f. Support is carried to the sheeting between the struts by horizontal structural members (wales).  The wale members should be designed to support a uniformly distributed load equal to the maximum pressure determined from figure 14-11 times the spacing between the wales.  The wales may be assumed to be simply supported (pinned) at the struts.

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The use of steep or vertical slopes for a deep excavation is often necessitated by land area availability or economics.

Such slopes are commonly supported by a cantilever wall (only for shallow excavations), a braced wall, or a
tieback wall (fig 14-10).  In some cases, it may be economical to mix systems,  such as a free slope and a
tieback wall or a tieback wall and a braced wall.  Table 14-1 summarizes the wall types with their typical properties and advantages and disadvantages.  Table 14-2 lists factors for selecting wall support system sfor a deep excavation (>20 feet).  Table 14-3 gives design parameters, such as factors of safety, heave problems, and supplemental references.

Figure 14-10.  Types of support systems for excavation.

Table 14-1.  Types of Walls

Table 14-2.  Factors Involved in Choice of a Support System for a Deep Excavation

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Common types of prestressed concrete piles are shown in "FIGURE 5.5 Typical prestressed concrete piles; dimensions in millimeters."  Prestressed piles are typically produced at a manufacturing plant and the first step is to set-up the form, which contains the prestressed strands that are surrounded by wire spirals. The concrete is then placed within the form and allowed to cure. Once the concrete has reached an adequate strength, the tensioning force is released, which induces a compressive stress into the pile. The prestressed piles are then loaded onto trucks, transported to the site, and stockpiled such as shown in Fig. 16.21.

Solid square concrete piles, such as shown in Fig. 16.21, are the most commonly used type of prestressed piles. As shown in Fig. 16.21, the end of the pile that will be driven into the ground is flush, while at the opposite end; the strands protrude from the concrete. The main advantage of prestressed concrete piles is that they can be manufactured to meet site conditions. For example, the prestressed concrete piles shown in Fig. 16.21 were manufactured to meet the following specifications: 12 in. (0.3 m) square piles

Design load = 70 tons (620 kN) per pile
Required prestress = 700 psi (5 MPa)
28 day compressive stress = 6000 psi (40 MPa)
Maximum water-cement ratio = 0.38
Portland cement type V (i.e., high sulfate content in the soil)

FIGURE 16.21 Prestressed concrete piles stockpiled at the job site.

Large pile-driving equipment, such as shown in Fig. 16.22, is required in order to drive the piles into place. If the piles are to be used as end-bearing piles and the depth to the bearing strata is variable, then the first step is to drive indicator piles. An indicator pile is essentially a prestressed pile that is manufactured so that it is longer than deemed necessary. For example, if the depth to adequate bearing material is believed to be at a depth of 30 ft (9 m), then an indicator pile could be manufactured so that it is 35 ft (11 m) long. Usually about 10 to 20 percent of the piles will be indicator piles. The indicator piles are used to confirm embedment conditions and thus some indicator piles may be driven near the locations of prior borings, while other indicator piles are driven in areas where there is uncertainty as to the depth of the bearing strata. Once the indicator piles have been driven, the remainder of the prestressed piles are manufactured with the lengths of the piles based on the depths to bearing strata as determined from the indicator piles.

FIGURE 16.22 Pile driving equipment. A prestressed concrete pile is in the process of being hoisted into position.

It is always desirable for the geotechnical engineer to observe the driving conditions for the prestressed piles. Prior to driving the piles, basic pile driving information should be recorded (see Table 16.6). In addition, during the actual driving of the piles, the number of blows per foot of penetration should be recorded. The pile-driving contractor typically marks the pile in one-foot increments so that the number of blows per foot can be easily counted.





Table 16.7 presents actual data during the driving of a prestressed pile. At this site, soft and liquefiable soil was encountered at a depth of about 15 to 30 ft (4.6 to 9.2 m) below ground surface.



Although the blows per foot at this depth were reduced to about one per foot, the driving contractor actually allowed the hammer to free fall and thus the energy supplied to the top of the pile was significantly less than at the other depths. For the data in Table 16.7, the very high blow counts recorded at a depth of 31 ft (9.5 m) are due to the presence of hard bedrock that underlies the soft and loose soil. Figure 16.23 shows the completed installation of the prestressed concrete pile. The wood block shown on the top of the concrete pile in Fig. 16.23 was used as a cushion in order to protect the pile top from being crushed during the driving operation.

FIGURE 16.23 A prestressed concrete pile has been successfully driven to the bearing strata. The wood block shown on the top of the concrete pile was used as a cushion in order to protect the pile top from being crushed during the driving operation.

A major disadvantage of prestressed concrete piles is that they can break during the driving process. The most common reason for the breakage of a prestressed concrete pile is because it strikes an underground obstruction, such as a boulder or large piece of debris that causes the pile to deflect laterally and break. For example, Fig. 16.24 shows the lateral deflection of a prestressed concrete pile as it was driven into the ground. In some cases, the fact that the pile has broken will be obvious.

FIGURE 16.24 Lateral displacement of a prestressed concrete pile during the driving operations.

In Fig. 16.25, the prestressed concrete pile hit an underground obstruction, displaced laterally and then broke near ground surface. In other cases where the pile breaks well below ground surface, the telltale signs will be a continued lateral drifting of the pile and low blow counts at the bearing strata.


If a pile should break during installation, the standard procedure is to install another pile adjacent to the broken pile. Often the new pile will be offset a distance of 5 ft (1.5 m) from the broken pile. Grade beams are often used to tie together the piles and thus the location of the new pile should be in-line with the proposed grade beam location. The structural engineer will need to redesign the grade beam for its longer span.

After the piles have been successfully installed, the next step is to construct the remainder of the foundation, as follows:  Construct the Remainder of the Foundation - Piles.

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The previous sections have described shallow foundation excavation and construction, foundation construction in open and braced excavations, and groundwater control. The next two sections will discuss the excavation and construction of deep foundations.

A common type of deep foundation support is through the use of piers and grade beams. The typical steps in the construction of a foundation consisting of piers and grade beams are as follows:

1. Excavation of piers. Figures 16.9 to 16.11 show the excavation of the piers using a truck-mounted
auger drill rig. This type of equipment can quickly and economically excavate the piers to the
desired depth. In Figs. 16.9 to 16.11, a 30 in. (0.76 m) diameter auger is being used to excavate
the pier holes.

FIGURE 16.9 Truck-mounted auger drill rig used to excavate piers.

FIGURE 16.10 Close-up of auger being pushed into the soil.

FIGURE 16.11 Close-up of auger being extracted from the ground with soil lodged within its groves.

2. Cleaning of the bottom of the excavation. Piers are often designed as end-bearing members. For example, there may be a loose or compressible upper soil zone with the piers excavated through this material and into competent material. The ideal situation is to have the groundwater table below the bottom of the piers. This will then allow for a visual inspection of the bottom of the pier excavation. Often an experienced driller will be able to clean out most of the bottom of the pier by quickly spinning the auger.

A light can then be lowered into the pier hole to observe the embedment conditions (i.e., see Fig. 16.12). A worker should not descend into the hole to clean out the bottom, but rather any loose material at the bottom of the pier should be pushed to one side and then scraped into a bucket lowered into the pier hole. If it is simply not possible to clean out the bottom of the pier, then the pier resistance could be based solely on skin friction in the bearing strata with the end-bearing resistance assumed to be equal to zero.

FIGURE 16.12 A light has been lowered to the bottom of the pier in order to observe embed- ment conditions.

 


3. Steel cage and concrete. Once the bottom of the pier hole has been cleaned, a steel reinforcement cage is lowered into the pier hole. Small concrete blocks can be used to position the steel cage within the hole. Care should be used when inserting the steel cage so that soil is not knocked off of the sides of the hole. Once the steel cage is in place, the hole is filled with concrete. Figure 16.13 shows the completion of the pier with the steel reinforcement extending out of the top of the pier.

FIGURE 16.13 The pier hole has been filled with concrete. The steel reinforcement from the pier will be attached to the steel reinforcement in the grade beam.

4. Grade beam construction. The next step is to construct the grade beams that span between the piers. Figure 16.14 shows the excavation of a grade beam between two piers. Figure 16.15 shows the installation of steel for the grade beam. Similar to the piers, small concrete blocks are used to position the steel reinforcement within the grade beam. A visqueen moisture barrier is visible on the left side of Fig. 16.15. Figure 16.16 shows a pier located at the corner of the building. The steel reinforcement from the grade beams is attached to the steel reinforcement from the piers.



Once the steel reinforcement is in place, the final step is to place the concrete for the grade beams.

Figure 16.17 shows the finished grade beams. The steel reinforcement protruding out of the grade beams will be attached to the steel reinforcement in the floor slab.

FIGURE 16.14 Excavation for the grade beam that will span between the two piers.

FIGURE 16.15 Steel reinforcement is being installed within the grade beam excavation.

FIGURE 16.16 Corner of the building where the steel reinforcement from the two grade beams has been attached to the steel reinforcement from the pier.

FIGURE 16.17 The concrete for the grade beams has been placed. The steel reinforcement from the grade beams will be attached to the steel reinforcement in the floor slab.

5. Floor slab. Before placement of the floor slab, a visqueen moisture barrier and a gravel capillary break should be installed. Then the steel reinforcement for the floor slab is laid out, such as shown in Fig. 16.18. Although not shown in Fig. 16.18, small concrete blocks will be used to elevate the steel reinforcement off the subgrade and the steel will be attached to the steel from the grade beams. The final step is to place the concrete for the floor slab. Figure 16.19 shows the completed floor slab.

FIGURE 16.18 Positioning of the steel reinforcement for the floor slab.

FIGURE 16.19 Concrete for the floor slab has been placed.

6. Columns. When designing the building, the steel columns that support the superstructure can be positioned directly over the center of the piers. For example, Fig. 16.20 shows the location where the bottom of a steel column is aligned with the top of a pier. A steel column having an attached base plate will be bolted to the concrete. Then the steel reinforcement from the pier (see Fig. 16.20) will be positioned around the bottom of the steel column. Once filled with concrete, the final product will be essentially a fixed-end column condition having a high lateral resistance.

FIGURE 16.20 Location where a steel column will be attached to the top of a pier.

The main advantage of this type of foundation is that there are no open joints or planes of weakness that can be exploited by soil movement or seismic shaking. The strength of the foundation is due to its monolithic construction, with the floor slab attached and supported by the grade beams, which are in turn anchored to the piers. In addition, the steel columns of the superstructure can be constructed so that they bear directly on top of the piers and have fixed end connections. This monolithic foundation and the solid connection between the steel columns and piers will enable the structure to resist soil movement or seismic shaking.

Usually the structural engineer will design this foundation system. The geotechnical engineer provides various design parameters, such as the estimated depth of the bearing strata, the allowable end-bearing resistance, allowable skin friction in the bearing material, allowable passive resistance of the bearing material, and any anticipated downdrag loads that could be induced on the piers if the upper loose or compressible soil should settle under its own weight or during the anticipated earthquake. The geotechnical engineer will also need to inspect the foundation during construction in order to confirm the embedment conditions of the piers.

This type of foundation can also be used to resist the effects of expansive soil. However, when dealing with expansive soils, it is often preferred to use a raised concrete slab or raised wood floor to elevate the first floor off of the expansive subgrade. Loads from the raised floor are transferred to the grade beams and then to the piers. To mitigate uplift loads on the grade beams, a layer of easily deformable material, such as Styrofoam can be placed beneath the grade beams.

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An open excavation is defined as an excavation that has stable and unsupported side slopes. Table 16.1 presents a discussion of the general factors that control the excavation stability and Table 16.2 The A and B coefficients as obtained by Reese and Matlock lists factors that control the stability of excavation slopes in some problem soils.

A braced excavation is defined as an excavation where the sides are supported by retaining structures. Figure 11.17 shows common types of retaining systems for braced excavations. Table 16.3 lists the design considerations for braced excavations and Table 16.4 indicates factors that are involved in the choice of a support system for a deep excavation.

Table 16.1 Typical values of n, for cohesive soils (Taken from Poulos and Davis, 1980)Añadir leyenda

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Introduction
There are many different types of excavations performed during the construction of a project. For example, soil may be excavated from the cut or borrow area and then used as fill (Chap. 15). Another example is the excavation of a shear key or buttress that will be used to stabilize a slope or landslide.

Other examples are excavations for the construction of foundations, which will be discussed in this part. An important aspect of the excavation may be groundwater control, which will be discussed later.

1  Shallow Footing Excavations   A shallow foundation is often selected when the structural load will not cause excessive settlement or lateral movement of the underlying soil layers. Shallow foundations are also...

2  Open and Braced Excavations   An open excavation is defined as an excavation that has stable and unsupported side slopes. Table 16.1 presents a discussion of the general factors that control the excavation...

3  Groundwater Control   Groundwater and seepage, including the lowering of the groundwater table by pumping from wells, has been discussed previously. Groundwater can cause or contribute to...

4  Pier and Grade Beam Support   The previous sections have described shallow foundation excavation and construction, foundation construction in open and braced excavations, and groundwater control. The next ...

5  Prestressed Concrete Piles   Common types of prestressed concrete piles are shown in "FIGURE 5.5 Typical prestressed concrete piles; dimensions in millimeters."  Prestressed piles are typically produced at a...

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INTRODUCTION
There are many types of services performed by the geotechnical engineer during the actual construction of the project. Examples of these types of services are as follows:

1. Excavation of the foundation. During the construction of the project, the geotechnical engineer will often be asked or required to review the foundation excavation. This type of service involves measuring the dimension of geotechnical elements (such as the depth and width of footings) to make sure that they conform to the requirements of the construction plans. This service is often performed at the same time as the field observation of the foundation bearing conditions, such as confirming the presence of dense soil or intact rock.

In many cases, field observations to confirm bearing conditions and check foundation dimensions will be required by the local building department. In addition, a letter indicating the outcome of the observations must be prepared by the engineer to satisfy the local building department. Building departments often refer to these types of reports as foundation inspection reports. The local building department often considers these reports to be so important that they may not issue a certificate of occupancy until the reports have been submitted and accepted.




Foundation excavations will be discussed later.

2. Field load or performance tests. There are numerous types of field load or performance tests.

For example, load tests are common for pile foundations and are used to determine their load carrying capacity. Field load tests will be discussed later.

Besides load tests, there can be all types of performance tests during construction that will need to be observed by the geotechnical engineer. An example is the field-testing of tieback anchors as discussed later and Table 11.3.



3. Underpinning. There are many different situations where a structure may need to be under-
pinned. Common reasons for underpinning include supporting a structure that is sinking or tilt-
ing due to ground subsidence or instability of the structure. Foundation underpinning will be
discussed later.

4. Observational method. There are numerous types of subsurface conditions that can lead to delays
and additional expenses during construction. If it is anticipated that there will be earth failure, ground subsidence, or groundwater seepage, then it is important that the geotechnical engineer plan for such site conditions. Earth movement and failure can affect all types of construction projects. For example, Fig. 16.1 shows a slope failure during the grading of a project. The process of grading can undermine the toe of a slope or surcharge the head of a slope, leading to a failure such as shown in Fig. 16.1. The observational method is one approach that can be used to anticipate such conditions and modify the design if needed during construction. The observational method will be discussed later.